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            <div><h3 class="title">
                <a name="asio.reference.async_read"></a><a class="link" href="async_read.html" title="async_read">async_read</a>
            </h3></div>
        </div>
    </div>
    <p>
        <a class="indexterm" name="asio.indexterm.async_read"></a>
    </p>
    <p>
        The <code class="computeroutput"><span class="identifier">async_read</span></code> function is
        a composed asynchronous operation that reads a certain amount of data from
        a stream before completion.
    </p>
    <p>
        Start an asynchronous operation to read a certain amount of data from a stream.
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="MutableBufferSequence.html"
                                             title="Mutable buffer sequence requirements">MutableBufferSequence</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload1.html"
                                                        title="async_read (1 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="keyword">const</span> <span class="identifier">MutableBufferSequence</span> <span
                class="special">&amp;</span> <span class="identifier">buffers</span><span class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="identifier">is_mutable_buffer_sequence</span><span
                class="special">&lt;</span> <span class="identifier">MutableBufferSequence</span> <span class="special">&gt;::</span><span
                class="identifier">value</span> <span class="special">&gt;::</span><span class="identifier">type</span>  <span
                class="special">=</span> <span class="number">0</span><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload1.html" title="async_read (1 of 8 overloads)">more...</a></em></span>

<span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="MutableBufferSequence.html"
                                             title="Mutable buffer sequence requirements">MutableBufferSequence</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="CompletionCondition.html"
                                             title="Completion condition requirements">CompletionCondition</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload2.html"
                                                        title="async_read (2 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="keyword">const</span> <span class="identifier">MutableBufferSequence</span> <span
                class="special">&amp;</span> <span class="identifier">buffers</span><span class="special">,</span>
    <span class="identifier">CompletionCondition</span> <span class="identifier">completion_condition</span><span
                class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="identifier">is_mutable_buffer_sequence</span><span
                class="special">&lt;</span> <span class="identifier">MutableBufferSequence</span> <span class="special">&gt;::</span><span
                class="identifier">value</span> <span class="special">&gt;::</span><span class="identifier">type</span>  <span
                class="special">=</span> <span class="number">0</span><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload2.html" title="async_read (2 of 8 overloads)">more...</a></em></span>

<span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="DynamicBuffer_v1.html"
                                             title="Dynamic buffer requirements (version 1)">DynamicBuffer_v1</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload3.html"
                                                        title="async_read (3 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="identifier">DynamicBuffer_v1</span> <span class="special">&amp;&amp;</span> <span class="identifier">buffers</span><span
                class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="identifier">is_dynamic_buffer_v1</span><span class="special">&lt;</span> <span
                class="keyword">typename</span> <span class="identifier">decay</span><span
                class="special">&lt;</span> <span class="identifier">DynamicBuffer_v1</span> <span class="special">&gt;::</span><span
                class="identifier">type</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span
                class="special">&gt;::</span><span class="identifier">type</span>  <span class="special">=</span> <span
                class="number">0</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="special">!</span><span
                class="identifier">is_dynamic_buffer_v2</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span
                class="identifier">decay</span><span class="special">&lt;</span> <span class="identifier">DynamicBuffer_v1</span> <span
                class="special">&gt;::</span><span class="identifier">type</span> <span
                class="special">&gt;::</span><span class="identifier">value</span> <span
                class="special">&gt;::</span><span class="identifier">type</span>  <span class="special">=</span> <span
                class="number">0</span><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload3.html" title="async_read (3 of 8 overloads)">more...</a></em></span>

<span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="DynamicBuffer_v1.html"
                                             title="Dynamic buffer requirements (version 1)">DynamicBuffer_v1</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="CompletionCondition.html"
                                             title="Completion condition requirements">CompletionCondition</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload4.html"
                                                        title="async_read (4 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="identifier">DynamicBuffer_v1</span> <span class="special">&amp;&amp;</span> <span class="identifier">buffers</span><span
                class="special">,</span>
    <span class="identifier">CompletionCondition</span> <span class="identifier">completion_condition</span><span
                class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="identifier">is_dynamic_buffer_v1</span><span class="special">&lt;</span> <span
                class="keyword">typename</span> <span class="identifier">decay</span><span
                class="special">&lt;</span> <span class="identifier">DynamicBuffer_v1</span> <span class="special">&gt;::</span><span
                class="identifier">type</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span
                class="special">&gt;::</span><span class="identifier">type</span>  <span class="special">=</span> <span
                class="number">0</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="special">!</span><span
                class="identifier">is_dynamic_buffer_v2</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span
                class="identifier">decay</span><span class="special">&lt;</span> <span class="identifier">DynamicBuffer_v1</span> <span
                class="special">&gt;::</span><span class="identifier">type</span> <span
                class="special">&gt;::</span><span class="identifier">value</span> <span
                class="special">&gt;::</span><span class="identifier">type</span>  <span class="special">=</span> <span
                class="number">0</span><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload4.html" title="async_read (4 of 8 overloads)">more...</a></em></span>

<span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Allocator</span><span class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload5.html"
                                                        title="async_read (5 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="identifier">basic_streambuf</span><span class="special">&lt;</span> <span
                class="identifier">Allocator</span> <span class="special">&gt;</span> <span class="special">&amp;</span> <span
                class="identifier">b</span><span class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload5.html" title="async_read (5 of 8 overloads)">more...</a></em></span>

<span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Allocator</span><span class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="CompletionCondition.html"
                                             title="Completion condition requirements">CompletionCondition</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload6.html"
                                                        title="async_read (6 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="identifier">basic_streambuf</span><span class="special">&lt;</span> <span
                class="identifier">Allocator</span> <span class="special">&gt;</span> <span class="special">&amp;</span> <span
                class="identifier">b</span><span class="special">,</span>
    <span class="identifier">CompletionCondition</span> <span class="identifier">completion_condition</span><span
                class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload6.html" title="async_read (6 of 8 overloads)">more...</a></em></span>

<span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="DynamicBuffer_v2.html"
                                             title="Dynamic buffer requirements (version 2)">DynamicBuffer_v2</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload7.html"
                                                        title="async_read (7 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="identifier">DynamicBuffer_v2</span> <span class="identifier">buffers</span><span
                class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="identifier">is_dynamic_buffer_v2</span><span class="special">&lt;</span> <span
                class="identifier">DynamicBuffer_v2</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span
                class="special">&gt;::</span><span class="identifier">type</span>  <span class="special">=</span> <span
                class="number">0</span><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload7.html" title="async_read (7 of 8 overloads)">more...</a></em></span>

<span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="AsyncReadStream.html"
                                             title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="DynamicBuffer_v2.html"
                                             title="Dynamic buffer requirements (version 2)">DynamicBuffer_v2</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="CompletionCondition.html"
                                             title="Completion condition requirements">CompletionCondition</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <a class="link" href="async_read/overload8.html"
                                                        title="async_read (8 of 8 overloads)">async_read</a><span
                class="special">(</span>
    <span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span
                class="special">,</span>
    <span class="identifier">DynamicBuffer_v2</span> <span class="identifier">buffers</span><span
                class="special">,</span>
    <span class="identifier">CompletionCondition</span> <span class="identifier">completion_condition</span><span
                class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">constraint</span><span
                class="special">&lt;</span> <span class="identifier">is_dynamic_buffer_v2</span><span class="special">&lt;</span> <span
                class="identifier">DynamicBuffer_v2</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span
                class="special">&gt;::</span><span class="identifier">type</span>  <span class="special">=</span> <span
                class="number">0</span><span class="special">);</span>
  <span class="emphasis"><em>» <a class="link" href="async_read/overload8.html" title="async_read (8 of 8 overloads)">more...</a></em></span>
</pre>
    <h5>
        <a name="asio.reference.async_read.h0"></a>
        <span><a name="asio.reference.async_read.requirements"></a></span><a class="link"
                                                                             href="async_read.html#asio.reference.async_read.requirements">Requirements</a>
    </h5>
    <p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">asio/read.hpp</code>
    </p>
    <p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
    </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%">
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            <div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
            </p>
            </div>
        </td>
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